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The present invention relates generally to accelerometers, and more specifically to silicon micro-machined convective accelerometers.
Silicon micro-machined accelerometers are known which employ the principle of free convection heat transfer of a hot air bubble in an enclosed chamber to provide a measure of acceleration. Such devices comprise a chamber micro-machined in a silicon substrate, across which is provided a heater resistor and on each side of which is disposed a thermocouple. During zero acceleration, the temperature profile about the heater resistor is symmetrical such that both thermocouples sense the same temperature and therefore provide the same output voltage. Acceleration applied along the thermocouple-heater-thermocouple axis causes disturbance of the temperature profile due to free convection heat transfer, thereby causing an asymmetrical temperature profile which is sensed by the thermocouples to provide output voltages that are different and a differential output voltage that is proportional to the applied acceleration. The differential output voltage typically requires signal conditioning to interface with the electronics of a particular application. Such signal conditioning is implemented using external electronic components and/or circuitry combined on the same substrate as the convective accelerometer.
Conventional silicon micro-machined convective accelerometers have drawbacks in that the sensitivity of these devices can vary. For example, the accelerometer sensitivity may vary with changes in the temperature of the silicon substrate and/or the local environment. The accelerometer sensitivity may also vary with changes in the power dissipated in the heater element.
It would therefore be desirable to have an improved silicon micro-machined convective accelerometer that is less sensitive to temperature and power fluctuations. Such an improved silicon micro-machined convective accelerometer would also have reduced power consumption.
In accordance with the present invention, a silicon micro-machined convective accelerometer is provided in which a thermal acceleration sensor and associated signal conditioning circuitry are included in a single monolithic device. The device is formed in a silicon substrate and is preferably implemented using standard CMOS processes. Integrated within the single monolithic device is the thermal acceleration sensor, heater control circuitry, an instrumentation amplifier, clock generation circuitry, voltage reference circuitry, a temperature sensor, and output amplifiers. The device can be packaged in a standard 8-pin integrated circuit package.
In one embodiment, an integrated convective accelerometer chip includes a convective acceleration sensor including a heater element and a pair of temperature sensing elements disposed on opposing sides thereof. The acceleration sensor is operative to produce a differential output voltage proportional to the magnitude of acceleration applied along an axis passing through the heater element and the pair of temperature sensing elements. The chip further includes amplification circuitry operative to extract an average output voltage from the differential output voltage produced by the acceleration sensor. The average output voltage provides a measure of the temperature gradient produced by the heater element. Still further, the chip includes control circuitry operative to produce a control output. In a preferred embodiment, the control output is a pulsed output voltage and operative to regulate the average output voltage, thereby regulating the temperature gradient produced by the heater element. Yet further, the chip includes a temperature sensor operative to produce a voltage level Proportional To the Absolute Temperature (PTAT) of the chip. Temperature compensation circuitry included on the chip or implemented externally to the chip can use the PTAT voltage level to compensate for changes in the temperature of the chip and/or the local environment.
In a preferred embodiment, the average output voltage and therefore the temperature gradient produced by the heater element are regulated by reading output voltages provided by the pair of temperature sensing elements, comparing these voltages to a reference voltage, producing a pulsed output voltage having a pulse density proportional to the magnitude of a desired output voltage, and regulating the average output voltage using the pulsed output voltage.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.